In ore concentrators of the type shown in U.S. Pat. No. 1,986,778 to Hinkley, the number of leads employed on the rotating bowl was between about five and ten, so that the size of the center hole to which material is led was minimal. Accordingly, hollow bowl rotating shafts could be used to conduct the material from the center hole and away from the bowl.
However, when the size of the machine increased significantly, the number of leads in the bowl increased to between 60 and 100, for example. As a consequence, the size of the central opening and of the hollow shaft increased markedly, and to the point where undesirably large shaft bearings and associated apparatus would be required.